1. Field of the Invention
This invention relates generally to conductive polymers. It relates particularly to semi-conductive tin dioxide-surfaced polyimide films with conductivities in the range of 1.0.times.10.sup.-2 to 3.0.times.10.sup.-3 ohm-1 and to the process for their preparation.
2. Description of the Related Art
A need exists in the aerospace industry for flexible, electrically conductive polymeric films and coatings having electrical conductivities in the semi-conducting range for use on large space structures to provide for relief from space-charging, and on advanced aircraft to provide for lightning strike resistance, and other applications. For successful use, these films and coatings should exhibit no significant loss of electrical or mechanical properties during exposure to elevated temperatures, as well as severe weather and other pertinent use conditions.
A procedure for making SnO.sub.2 coated polyimide films via incorporation of tin compounds was reported (Taylor, L. T.; and St. Clair, A. K.: Polyimides, vol. 2, edited by K. L. Mittal, Plenum Publishing Corp., New York, 1984, pp. 617-645. Ezzell, S. A.; and Taylor, L. T.: Macromolecules, vol. 17, 1984, pp. 1627-1632.). Synthesis of these materials involved preparation of a polyamic acid from an aromatic diamine and dianhydride in a polar aprotic solvent, addition of a soluble tin compound (Tin(II) chloride dihydrate or dibutyltin dichloride) to the polyamic acid, preparation of a film, and thermal curing to 300.degree. C. in air. This procedure produced materials with one conductive surface, viz., the side facing up (air side) during thermal curing. These films had an air-side surface conductivity of 4.5.times.10.sup.-5 ohm.sup.-1. Characterization of these films revealed the conductive properties to be due to a layer of tin dioxide formed on the surface of the film from migration and decomposition of the tin compound during thermal treatment.
Other researchers in related work (Goldstein, R. D.; Brown, E. M.; and Maldoon, L. C.: Trans. Nucl. Sci, NS-29, 1082, pp. 1621-1628) evaluated polyimide film coated with sputtered indium tin oxide (ITO) for thermal control coating applications. Electrical integrity of the ITO was monitored through abrasion, adhesion, thermal cycling, flexure, and other relevant tests. Test results varied depending on the thickness of the ITO layer, and its subsequent conductivity. A sample measuring 6.04.times.10.sup.-3 ohm.sup.-1 exhibited negligible conductivity change after abrasion and adhesion testing. However, flexural testing of the film caused a 60% decrease in conductivity which eliminated its usefulness. Thermal cycling determined the electrical properties of the ITO coating to be stable to 125.degree. C., above which large decreases in electrical conductivity were observed.
It is a primary object of the present invention to produce polyimide films having surface conductivities in the range of 1.0.times.10.sup.-2 to 3.0.times.10.sup.-3 ohm.sup.-1.
Another object of the present invention is to provide a controlled process for the production of polyimide films having surface conductivities in the range of 1.0.times.10.sup.-2 to 3.0.times.10.sup.-3 ohm.sup.-1.